Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention (e.g., Patent Document 1).
Furthermore, for example, Patent Document 2 discloses a semiconductor device in which, to reduce oxygen vacancy in an oxide semiconductor layer where a channel is formed, an insulating film which releases oxygen by heating is used as a base insulating layer of the oxide semiconductor layer.